The present invention is with respect to an instrument for measuring the deforming capacity of erythrocytes having at least one sample vessel that is walled off by a foil into two vessel spaces, the foil having a sample flow opening, whose diameter is less than the major quiescent diameter of an erythrocyte, a first one of said vessel spaces being designed to take up a buffer solution together with erythrocytes so that there is a pressure gradient between the two vessel spaces towards the second vessel space (in which there is buffer solution free of erythrocytes) producing a flow between the first and second spaces through the said opening.
An important parameter for the flow behavior of blood is the capacity of the erythrocytes to undergo deformation. In the large blood vessels this deformation is an adaptation of the erythrocytes to the hydrodynamic forces, that is to say to make the hydrodynamic resistance as low a possible, while in small blood vessels it is a question of adaptation to the geometrical limitations. In the last few years wide-ranging basic research has been undertaken on the cause and effect of the deformation capacity of the erythrocytes, which do not have nuclei, of the blood of man and other mammals, such work having made it clear that the normally high ability to flow of the blood at a high speed through the finest blood vessels is dependent on the deformation capacity. In this respect the deformation capacity is greatly dependent on the mechanical properties of the erythrocytes, such properties in turn being controlled by structure and chemical make-up thereof.
An erythrocyte may be looked upon as a membrane sac, that is incompletely filled with liquid and whose size is species-specific and is not dependent in any way on the size of the body. The mean diameter of the humane erythrocytes is 7.5 microns with a height of about 1.5 to 2 microns. The volume is 85 to 90 cubic microns and the surface area somewhere between 120 and 160 square microns. On average there are about 25 trillion erythrocytes present in the circulating blood, that have an overall surface area of 3000 to 4000 square meters.
If one were to take it that an erythrocyte has a volume of 90 cubic microns and a spherical form, the ratio between the surface area and the volume of the erythrocytes would then be about 1.07. However the real ratio is between 1.3 and 1.8. Because of the deformation capacity of the membrane sac and the viscosity of the liquid therein, an erythrocyte is in a very good position with respect to changing its outer form in keeping with the forces acting on it. The main purpose of the erythrocytes is the transport of oxygen to the ultimate user, that is to say every parenchyma cell, and to take up carbon dioxide as a product of metabolism from such cells. This purpose may only be effected by the erythrocytes if they are in a position to make their way along the nutritive capillary bed with its tubes that have a diameter of 3 to 5 microns, that is to say smaller than the diameter of the erythrocytes themselves within a reasonable time. On the footing of this simple geometrical limiting condition one may at once see the importance of the deformation capacity of the erythrocytes with respect to the flow properties in the nutritive capillaries in all the organs of the body. Different diseases have effects on the deformation capacity of the erythrocytes so that measuring the properties of erythrocytes with a limited deformation capacity may make diagnosis of disease processes possible.
The nutritive capillaries and more specially the splenic sinuses are responsible for filtering out erythrocytes no longer having a good deformation capacity so that the time that these erythrocytes are kept in the blood circulation is quite limited. It may be seen from that only a specially sensitive measuring method and measuring instrument would have the power of sensing the minute or discrete changes in the deformation capacity of erythrocytes. On the one hand, when measuring the deformation capacity or deformability, one purpose is that of copying the geometry and the hydrodynamics of the microcirculation as far as possible while, on the other hand, seeing that the measuring operation itself has no or only the least possible effect on the simulation of microcirculation conditions.